The present invention relates to light emitting diodes and in particular relates to such diodes that incorporate Group III nitride active layers on transparent substrates, such as silicon carbide, in an orientation in which the active layers are adjacent the mounting structure while the structural substrate forms the emitting face of the diode. Such an arrangement is often referred to as a “flip chip” light emitting diode.
Light emitting diodes (“LEDs”) are one type of photonic devices; i.e. those in which a forward current generates light (photons), or in which photons generate a forward current (photodetectors). Because light emitting diodes are solid-state devices, they share the long lifetime, high reliability, and robust physical characteristics of many other semiconductor devices. Additionally, light emitting diodes that can produce white light are becoming commercially more common, with the potential to compete with or replace many types of existing artificial lighting (e.g., incandescent, florescent, vapor).
Silicon carbide and the Group III nitrides (i.e., binary, ternary and quaternary compounds of Ga, Al and In with nitrogen) are semiconductor compounds of significant interest for LEDs because their wide bandgaps enable them to generate higher frequency photons under forward current. These higher frequencies are in turn represented by the green, blue, violet, and ultraviolet portions of the electromagnetic spectrum. Accordingly, such diodes can be combined with lower frequency red and yellow diodes to produce a combination of frequencies that together produce white light. Alternatively, they can be used to excite phosphors which emit colors (typically yellow) that together with the blue emission produce white light.
As between silicon carbide (SiC) and the Group III nitrides, the Group III nitrides are becoming more preferred for the active portions of wider-bandgap LEDs because the wavelengths at which they emit can be tuned to some extent by the atomic composition of the nitride, and because they are direct rather than in direct emitters. Silicon carbide nevertheless provides a useful substrate material for Group III nitrides because it can be conductively doped, is physically, chemically and thermally robust, can be formed to be transparent, and provides a suitable crystal lattice match for the nitride compositions.
From a practical standpoint, an LED's useful emission is best understood and measured by the amount of light that actually leaves the device and can be externally perceived. Stated differently, photons generated by the active layers (junction) in a device are initiated in all directions. Accordingly, maximizing the number of these photons that actually exit the device in the direction of the desired transmission of light is a practical goal.
Because of the well-understood implication of Snell's law, photons reaching the surface (interface) between the semiconductor material and the surrounding atmosphere will be either refracted or internally reflected. If internally reflected repeatedly, the photons are eventually reabsorbed and never offer visible light that leaves the device.
Therefore, in order to maximize the opportunity for photons to exit an LED, particularly flip-chip devices in which the transparent silicon carbide substrate represents the exposed surface, it has been found useful to pattern the silicon carbide into geometric shapes that increase the probability of refraction over internal reflection, and thus enhance light extraction. Exemplary (but not limiting) techniques and structures are set forth in commonly assigned U.S. patents and co-pending U.S. published applications U.S. Pat. Nos. 6,888,167; 6,821,804; 6,791,119; 6,747,298; 6,657,236; 20050194603; 20050194584; 2005151138; the contents of which are incorporated entirely herein by reference.
Other current methods for improving light extraction include lapping or otherwise mechanically roughening the appropriate surfaces, or bevel-cutting the chip. Nevertheless, such mechanical methods tend to induce or introduce stress into the material and can increase wafer breakage to an unacceptable extent, particularly on relatively thin wafers; e.g. those thinner than about 125 microns. Mechanical methods are also limited in terms of the position in the fabrication sequence in which they can be employed. Bevel cutting is slow and requires special diamond saw blades that are relatively expensive and also tend to reduce yield.
The practical employment of lens-type (lenticular) structures has remained limited, however, because the techniques for producing the structures typically include complex, multi-step photo processes, or the use of complex gray tone lithography masks to pattern resist layers. Other techniques create pillars in the resist using standard photolithography, following which the pillars are reheated (reflowed) to create spherical-shaped structures.
Nevertheless, these techniques tend to be impractical for high-volume manufacturing and are of more limited use with silicon carbide or other material systems that do not etch at a 1:1 ratio to a polymer in a dry etch transfer process. When the etch rate selectivity is below 1:1 (substrate:resist) the resulting features will be flattened or reduced in radius as compared to the original pattern. This in turn reduces the light extraction efficiency of the resulting surface because of the deviation from desired or targeted critical angles.
As another factor, in order for the lenticular features to be most useful, they should be positioned as close as possible to the emitting layers of the LED. All other factors being equal, a thicker non-lenticular layer, even if transparent, tends to increase the probability of undesired internal absorption or reflection.
Accordingly, a need exists for improved techniques for producing shaped (lens, lenticular) features in silicon carbide for incorporation in flip chip type light emitting diodes.